DESCRIPTION The specific neuronal circuitry that provides the foundation for sleep-related electrical activities in the brain is composed of reciprocal loops between interconnected areas of thalamus and neocortex. There is emerging evidence that this circuitry can be compromised in humans with generalized absence epilepsy, with the result that normal oscillatory sleep rhythms are converted into pathological epileptic activities. Within the thalamic half of the thalamocortical loop synaptic mechanisms promote both normal and abnormal oscillatory patterns. It is our hypothesis that regulation of synaptic strength within the thalamus is a key factor in determining whether normal are transformed into those associated with absence epilepsy. As a first step in testing this hypothesis, presynaptic control mechanisms in the thalamus will be investigated. These presynaptic mechanisms are expected to reduce the efficacy of synaptic transmission and may play a key role in the prevention of the neuronal activity associated with absence seizures. Gamma-aminobutyric acid (GABA receptor agonists and antagonists are powerful regulators of experimental absence seizures. The presynaptic actions of these compounds will be examined in the two principle types of neurons in rat somatosensory thalamus, the inhibitory cells within nucleus Reticularis thalami (nRt) and the excitatory relay neurons in the ventrobasal complex (VB). A microscope fitted with differential interference optics will be used to obtain high quality whole-cell voltage-clamp recordings form neurons visualized in thalamic slices maintained in vitro. The results of these studies may lead to new approaches in the treatment of human generalized epilepsy.